1. Field of the Invention
The present invention relates to a process for reducing oxidic hydrogenation catalysts and to a process for hydrogenating aldehydes over the reduced catalysts to form alcohols. More particularly, the invention relates to the alcohols prepared by the hydrogenation reaction.
2. Discussion of the Background
Alcohols can be prepared by catalytic hydrogenation of aldehydes which, for example, have been obtained by hydroformylation of olefins or by the aldol condensation reaction. Large amounts of alcohols are used as solvents and as intermediates for the preparation of many organic compounds. Important downstream products of alcohols are plasticizers and detergents.
It is known that aldehydes can be reduced catalytically with hydrogen to form alcohols. In such reduction reactions, catalysts which comprise at least one metal selected from groups 1b, 2b, 6b, 7b and/or 8 of the Periodic Table of the Elements are frequently employed. The hydrogenation of aldehydes can be conducted continuously or batchwise in the gas or liquid phase using pulverulent or shaped catalysts. For the industrial preparation of alcohols by hydrogenation of aldehydes, preference is given, especially in the case of high-volume products, to continuous processes using fixed-bed catalysts in the gas or liquid phase. Compared to gas-phase hydrogenation, liquid-phase hydrogenation has the more favorable energy balance. This advantage increases with increasing molar weight of the aldehyde to be hydrogenated. Higher aldehydes having more than 7 carbon atoms are, therefore, preferably hydrogenated in the liquid phase.
The hydrogenation catalysts normally used in industry are produced by reduction of appropriate precursors which contain the catalytically active metals in oxidic or salt-like form. Hydrogenation catalysts which have been activated in this manner are very reactive and are rapidly oxidized in air; some are even pyrophoric.
The reduction of the catalyst precursors should, in order to keep the start-up times or down-times of a hydrogenation reactor as short as possible, occur as quickly as possible but without impairing the activity and/or operating life of the catalyst.
The reduction of catalyst precursors arranged in a fixed bed to give the actual active metal-containing catalyst species is a step which has a critical influence on the success of the subsequent hydrogenation process. During the reduction of the metal compounds present in the catalyst precursor, heat is liberated. This amounts, for example, to 20 kcal/mol in the reduction of CuO to Cu. Overheating of the precursor or the catalyst during the reduction has to be prevented because otherwise thermal damage to the catalyst, e.g. enlargement of the metal crystallites, occurs. In order to obtain a hydrogenation catalyst having an optimum activity and a high strength, the reduction has to be conducted under extremely careful temperature control.
The reduction of catalyst precursors can be conducted in a liquid phase or a gas mixture.
In the absence of a liquid phase, the catalyst precursor is frequently reduced with a gas mixture consisting of hydrogen and an inert gas, preferably nitrogen. The gas-phase reduction of a copper catalyst is described in JP 61-161146. Reduction of Cu/Zn catalysts is known as disclosed in DE 17 68 313 and DE 34 43 277. JP 1-127042 discloses the gas-phase reduction of Cu/Cr catalysts.
Because of the low heat capacity of gas mixtures, reduction can be conducted only slowly and/or a very high gas hourly space velocity (GHSV) has to be set to ensure removal of heat. Reduction using a high gas hourly space velocity produces large amounts of gas which have to be disposed of. Because of the high inert gas content of the gas mixture, working-up of the gas to recover hydrogen is not worthwhile. The high gas hourly space velocity can be achieved more economically by circulating the gas with the aid of a circulating gas blower.
As an alternative to gas-phase reduction, the catalyst precursor can be reduced in the presence of a liquid phase. Here, the heat of reduction is removed by means of the liquid.
As disclosed in GB 385 625, a Cu/Cr catalyst precursor is reduced in the presence of a carboxylic ester at a liquid hourly space velocity (LHSV) of 8 h.sup.-1. JP 47-14113 describes the activation of a Cu/Cr catalyst in a stream of lactone (LHSV=0.67 h.sup.-1) at 200.degree. C. Another method of reducing Ni/Cu/Mo or Co/Cu catalyst precursors is disclosed in DE 35 24 330. Here, reduction is conducted at 200.degree. C. and 250 bar in a stream of isopropanol at a throughput per unit cross-sectional area of 60 m.sup.3 /(m.sup.2.multidot.h) for an Mo-containing catalyst and of 30 m.sup.3 /(m.sup.2.multidot.h) for a Co-containing catalyst.
These reduction processes have the disadvantage that they employ high temperature and/or pressures and require long reaction times up to a number of days.
Still another process for reducing hydrogenation catalysts is described in EP 0 689 477. Here, copper-containing catalyst precursors, in particular Cu/Cr, Cu/Zn, Cu/Fe, Cu/Al and Cu/SiO.sub.2, are reduced in the liquid phase in a two-stage process. The liquid phase can comprise esters, alcohols or hydrocarbons. The two-stage process of EP 0 689 477 employs a temperature program in which at least 10% by weight of the copper is reduced in the first stage at a temperature ranging from 20 to 140.degree. C. In the second stage, the temperature is increased from 140 to 250.degree. C. Disadvantages are the complicated temperature control and the still long reduction time of, on average, 40 hours. A need continues to exist for a method of effectively reducing catalyst precursors without impairing the resultant catalyst.